This invention relates to a bearing assembly and, more particularly, to an improved electrically and thermally conducting high speed bearing assembly particularly adapted for electrical high power tubes such as electrical X-ray generating tubes.
X-ray tubes generally comprise a cathode thermionic emitter and an axially spaced anode target positioned in an evacuated tube. Cathode and target are connected in an electrical circuit so that the cathode is electrically heated to emit a stream or beam of electrons directed to the anode target. The electron beam is appropriately focused as a thin beam of very high velocity electrons striking the anode target. X-ray targets utilize a striking surface of predetermined materials (usually a refractory metal) and a particular geometric shape so that the kinetic energy of the striking electron beam is converted to electromagnetic waves of high frequency, i.e. X-rays which emanate from the target to be collimated and focused for penetration into an object for well known x-ray medical diagnostic examination or material analysis procedures. The high velocity electron beam striking the anode target generates extremely high and localized temperatures in the anode target leading to early deterioration of the target structure, particularly composite targets. For this reason anode targets are adapted to rotate so that the impinging electron beam does not concentrate at a small region of the target area for a delayed period of time which could result in a rapid and excessive temperature rise and delay rapid distribution of heat throughout the target structure. As a consequence of progress to higher power x-ray tubes and their concurrent need for greater target heat dissipation, it has become a practice to utilize increasingly higher rotating speed anode target assemblies. A rotating anode x-ray target generally comprises a rotating shaft supported disk-like anode having a concentric annular band of a target metal thereon which is impinged by the electron beam from the cathode. In the enclosed and evacuated environment of an X-ray tube, the target structure must have a high heat storage capacity since most heat transfer from the target takes place essentially through radiation from the target to the x-ray tube structure. The practice of rotating targets has progressed to target rotational speeds exceeding 10,000 rpm. for satisfactory heat distribution and radiation. At such speeds in an adverse environment the bearing assembly for the anode target is required to be of high precision low friction and low wear to maintain effective and optimum longer term operating conditions. Bearing wear leads to high frequency vibration and unacceptable noise levels. In addition, the high voltages required for the cathode and anode operation and the presence of the electron beam leads to the development of high voltage potentials in the target anode bearing assembly with concurrent undesirable electrical arcing and welding.